Electronic effects of the substituent on the dioxygen-activating abilities of substituted iron tetraphenylporphyrins: a theoretical study.

A density functional theoretical (DFT) method was applied to understand the effects of the substituent on dioxygen activation by a series of substituted iron tetraphenylporphyrins [FeT(o/p-R)PP (o = ortho-substituted, p = para-substituted; R = -H, -Cl, -NO2, -CH3, -OCH3)]. The ground states (GS) of the dioxygen adducts of the substituted iron tetraphenylporphyrins [FeT(o/p-R)PPO2] were determined at the B3LYP/6-31G(d) level without any symmetry constraints. Binding energy calculations indicated that the presence of electron-withdrawing substituents at the para position favors O2 binding. Calculations of the O-O bond length of the adsorbed dioxygen revealed that the influence of the p-substituent on the activation of dioxygen decreases in the order p-CH3 > p-Cl > p-OCH3 > -H > p-NO2, while the influence of the o-substituent decreases in the order o-NO2 > o-CH3 > o-Cl > o-OCH3 > -H. The low-lying excited states (LLES) of the FeT(o/p-R)PPO2 adducts suggest that the ability to activate dioxygen decreases in the order o-CH3 > o-OCH3 = -H > o-NO2 > o-Cl for o-substituents and p-CH3 > p-Cl > -H > p-NO2 > p-OCH3 for p-substituents. NBO charge population analysis and spin density analysis showed that substitution caused more β-electrons to be transferred from the iron tetraphenylporphyrin to the dioxygen, which enhanced dioxygen activation. Spin density analysis confirmed that the β-electron population at the adsorbed dioxygen is an accurate indicator of the degree of dioxygen activation. The trend observed in porphyrin catalytic activity as the substituent on the dioxygen adduct was varied is consistent with the trend in the binding energy. It is clear that substituents at the ortho and para positions in these dioxygen adducts play different roles in dioxygen activation.